Liquid heater especially adapted for liquid submerged use

ABSTRACT

The invention relates to a liquid heater, especially adapted for liquid submerged uses, as for example, for heating a swimming pool. The heater has a mixing unit for air and fuel with a venturi configuration to permit the use of air under comparatively low pressures. The combustion generated is of the pulse type and the combustion chamber in which the fuel-air mixture is ignited has a body of material of high radiating potential, such as ceramic, which is heated in the combustion chamber and which radiates its heat to the enclosing heatconducting walls of the chamber in contact with the liquid to be heated. An exhaust tube also submerged in the liquid to be heated carries the exhaust gases to a region near the surface of the liquid to be heated and discharges it into the liquid causing the liquid to splash against part of a discharge device at the outlet of the exhaust tube made of heat-conducting material, such as metal, to extract thermal energy from said exhaust gases.

United States Patent [72] Inventors Francis M.Whitacre 148 Shoreview Road, Manhasset, N.Y.

451 Patented Jan.12,1971

[S4] LIQUID HEATER ESPECIALLY ADAPTED FOR LIQUID SUBMERGED USE 12 Claims, 4 Drawing Figs.

[52] U.S.Cl.

43I/l,43l/347,43l /354 [51] lnt.Cl. F24h l/20 FOREIGN PATENTS 430,678 6/ I 935, Great Britain 431/354 687,303 2/1953 Great Britain... l 26/ 360 Primary Examiner-Frederick L. Matteson Assistant Examiner-Robert A. Dua Attorney-Burgess, Ryan & Hicks ABSTRACT: The invention relates to a liquid heater, especially adapted for liquid submerged uses, as for example, for heating a swimming pool. The heater has a mixing unit for air and fuel with a venturi configuration to permit the use of air under comparatively low pressures. The combustion generated is of the pulse type and the combustion chamber in which the fuel-air mixture is ignited has a body of material of high radiating potential, such as ceramic, which is heated in the combustion chamber and which radiates its heat to the enclosing heat-conducting walls of the chamber in contact with the liquid to be heated. An exhaust tube also submerged in the liquid to be heated carries the exhaust gases to a region .near the surface of the liquid to be heated and discharges it into the liquid causing the liquid to splash against part of a discharge device at the outlet of the exhaust tube made of heat-conducting material, such as metal, to extract thermal energy from said exhaust gases.

GAS

PATENTED JAN 1 2 I97! SHEET 1 BF 2 INVENTORS FRANCIS M. WHITACRE JOHN L. UMPLEBY ATTORNEYS HPATENTEDJANIZISTI 3554.182

" SHEET 2 0F 2 FIG. 2 47 lOc INVENTORS v FRANCIS M.WH|TACRE JOHN L. UMPLEBY LIQUID HEATER asiiizcmrv human m LIQUID, suama tcanuss I I The present invention relates to a liquid heater and, although the heater has a wide range of utility, it is particularly useful for heating the water in a swimming pool.

One object of the present invention is to' provide a new and improved comparatively inexpensive heater for swimming pools or other liquid-heatingapplications, which is. compact and highly efficient and which operates" at comparatively low ln accordance with certain features of the present invention, the heater is of the combustion type in Twhichffuel-air mixtures are created and ignited, and.'resulting combustion is controlled to produce successive combustion pulses, resulting in high heat release in relatively 'small'spaceand the scrubbing of the heat transfer wallsof the combustion chamber substantially free from insulating deposits resistant to heat transfer.

As another feature of the present invention, the fuel-air .mixing systemand the-ignition system arelso combined,

designed and correlated as to eliniinateoratleast minimize substantially flashbacksor backfiring andto is insure thorough fuel-air mixing before ignition,

As a further feature 'the. presentinvention, the chi s". bustion chamber is designed topromote surfacefcombustion tive' heat transfer, (3 )submergedfcombustion beneath the surface of .the watery or "other liquid-to be heated-and (4) a venturi tube. For that purpose, the mixing tube 22 comprises an outer cylindrical standard pipe 23 of suitable metal, such as standard steel, copper or bronze, having an internal ceramic lining 24 of circular cross sections, formed or cast with venturi produce a combustible mixture.

boundaries converging in opposite directions towards a throat region 25. The fuel feed pipe 21 extends vertically in operative position of the heater, and its lower end'is centrally located in the mixing tube 22 and is closed except'for a series of holes or ports 26 in the throat region 25 of the mixing tube, through which the fuel, gas is discharged intofs'aid region. The air is delivered under pressure into the mixing tube 22 through the pipe t'ee inlet l6'bymeans of an inexpensive blower, such as that employed'in connec'tionwith vacuum cleaners, and as this air passes through the venturi throat region 25 of the mixing tube 22'at high velocity, the fuel is injected from-the fuel feed pipe 21 through its discharge ports 26 into said region. The agitation in the throat region 25 of the venturi mixing tube 22 and the high'velocity of the'air in this region, not only assures thorough and rapid mixing of the fuel and air but also prevents backflashes. The proportions of fuel an'd'air are such as to j For ignitionpurposes, t'here is 7 connected integrally or otherwise to' the lower end of .the fuel feed pipe 21 the upper end of an upright electrode ro'd or wire '27, made of suitable electrode metal, such-as-Nichrome .(a' ferrous alloy containing nickel and chrome) extending. centrally: through'the fuel-air 'inixingtube 22. Thelower end section ofthe electrode rod or wire 27 passes through a grounded screen grid 28-, extending across the bottom of the fuel-air mixing tube 22 and across the discharge of the combustion exhaustthroug'hfthe' water or other liquid being heated in the manner to recoversubstantial amounts of residual" thermal energy'from said exhaust without polluting the liquid to any significant extent. Various other objects, features and advantages of the present invention are; apparent from the following description and from the accompanying drawings," in which:

FIG. 1 is vertical section through a'heater constituting an embodimentof the present invention and shown inoperative position in a swimming pool for heating the water therein;

FIGS. 2 and 3 are sections of the heating device taken on the lines 2-2 and 3-3 respectively of FIG.- 1 but shown on a larg'erscalemnd i FIG. 4 is a vertical section through a modified form of fuelair mixing and igniting device for the heater, constituting another embodiment of the present invention.

Referring to FIGSJ, 2 and 3 of the drawings, there is shown a liquid heater of the present invention, comprising in general a fuel-air mixing'device '10, forming'part of a burner, an igni tion device 11 for the mixture produced in said mixing device screen grid to form an annular spark gap between the" electrode'rod and the screen grid. This spacing is accomplished by means of a ceramic insulating spacer 30, through which the and forming the other part of said burner, a combustion the water in a swimming pool, and is shown in upright positiontherein with the fuel-air mixing device 10 extending upwardly above the surface B of the water and 'the'combusti'on chamber 12 submerged in said water.

The fuel-air mixing device 10 in the embodiment of the in electrode rod 27 extends with a snug" fit. A current discharge athigh voltage for initial ignition passes through the fuel feed pipe 21 to the electrode rod 27 and for that purpose, the fuel feed pipe is made of conductive material. After initial ignition, no further current is required,since ignition is maintained by the thermal energy retained in the system. The screen grid 28 serves not only as a ground connection for sparking system, but also serves as a flashback barrier to prevent propagation of a possible flame into the fuel-air mixing chamber.

The combustion chamber 12 comprises a cylindrical tube 32 of heat conductive material, such as stainless steel, copper or brass, connected atits upper endto'the lower end of the pipe 23 forming the outer shell of the mixing tube 22. Inside this combustion chamber 12 is a center core 33, which is in the form of a solid cylindrical rod, as shown, or which may be in the form of a tube if desired, and which is located at a distance from the peripheral walls of said chamber to provide around said core an annular space 34 where the combustion of chamber by a tee 36 and may be anchored at its lower end by a bracket (not shown), or fixed by a self-support.

To prevent quenching of combustion in the headspace 35 of the combustion chamber 12, so that the-energy derived from vention shown in FIG. 1, comprises a standardpipe tee 15 having a center inlet 16, and two aligned branches or limbs 17 and 18, the outer branch 17 beingsealably closed by a plug 20, made of electrically insulating material, such as plastic or ceramic, and having a hole through which extends a feed pipe 21 for a fuel gas, such as propane. This fuel feed pipe 21 extends through the full length of the pipe tee l5, and beyond the outlet branch 18 of said tee, This outlet tee branch 18 is connected to the upper end of a mixing-tube 22, which for the purpose of attaining good mixing of air and fuel, is designed as this combustion is directed with maximum effect towards and alongside of the core 33, there is provided around this headspace and coaxial therewith, a cylindrical tubular screen 37 of heat resistant material, such as ceramic, or metal such as Nichrome or lnconel (chrome-nickel steel); when this c screen 37 is of metal, it may be in the form of a perforated or expanded sheet or a wire coil. The screen 37 is close to, but is equally spaced around its periphery from the peripheral wall a of. the combustion chamber tube 32 and extends from the screen grid 28 down to the top of the core 33 or to regions just space 35 of the combustion chamber 12 through repeated cycles. The combustion initiated in this headspace 35 is continued in the annular space 34 surrounding the core 33 over the surface of said core and is completed in said space. This combustion operation serves to heat the core 33 to an extremely high radiating temperature.

The annular space 34 around the core 33 in conjunction with the ignition and initial combustion headspace 35 and the exhaust tubes I3 immediately following all conjointly contribute to the establishment of pulsating waves having the effect of a hydrodynamic valve reducing backfiring and assisting in venting the products of combustion towards the exhaust exits of the system.

The advantages of the pulse combustion described are as follows:

I. Improved combustion is effected due to the increased pressure waves generated.

2. The increased velocity attained at the peak of pulsations, and the shock waves generated have the effect of scrubbing away the insulating static film of combustion products deposited on the inner surface of the peripheral walls of the combustion chamber 12, thereby minimizing one of the greatest resistances to heat transfer through these walls.

3. The transmittal ofkinetic energy at the peaks ofthe pulseeaswellasthe increaseddissipationoftherrnal energy from the mus ofcompressed gas to the central core 33 increases the radiation potential of said core.

Thenoise emanating from the heater by the combustion Experiments with a prototype having the dimensions described above and having a combustion chamber 12 in which its encasing tube 32 is 2 inches in internal diameter, indicate that the heater of the present invention will operate at an efficiency of about 82 percent with the core 33, and at an efficiency of about 25 percent without said core.

In the operation of the heater so far described, the com bustion in the combustion chamber 12 and over and along the surface of the core 33, heats this core to a very high temperature approaching incandesence. The energy absorbed by this core 33 is radiated to the peripheral walls of the combustion chamber 12 in contact with the water to be heated, to heat this water by heat transfer through these walls. In addition, some of the heat from the combustion in the chamber 12 is trans mitted by convection to the peripheral walls of said chamber. The exhausts from this combustion chamber 12 still contains substantial thermal energy and this is recovered for waterheating purposes by the exhaust system employed.

In the specific form shown, the exhaust system comprises the two exhaust tubes 13, desirably made of the same heatconducting metal as that of tube 32 encasing the combustion I chamber 12 and equally spaced from said tube 32. These exhaust tubes 13 are connected at the bottom to the bottom of the tube 32 by the T-union 36. As the exhaust gases from the combustion chamber 12 discharge into the exhaust tubes 13 and pass upwardly therealong, residual energy in these gases is transmitted to the walls of said tubes mainly by convection, thereby heating the water surrounding said tubes. The balanced configurations of these exhaust tubes 13 in relation to the combustion chamber 12 and their lengths and diameters are such as to maintain and regularize the combustion pulsapulses is muffled to an acceptable level by the submergence of the combustion chamber 12 in the water to be heated.

When the heater is operated with the exhaust under the water surface, increased back pressure results. If the. water surface fluctuates due to the turbulence created by the exhaust, this may interrupt the pulsating cycles. To overcome this, a catalyst 40 is applied to the core 33. Should the combustion then be interrupted, the catalyst 40 reignites the airi'uel mixture and normal pulsation is thereby reestablished. The catalyst can consist essentially of powdered iron.

The basic function of the core 33 is to provide a source of radiant energy to be transmitted to the peripheral wall of the combustion chamber 12 in accordance with the Stephan- Boltzman principle, which states: H

A L i [(10o) (100) where =Rate of heat transfer in B.t.u./ (B.t.u. h.)

=Area of core in sq. ft. E= Emissivity of core T =-Temperature of core in degrees Rankin Tgfigetperature of receiving surface in degrees Therefore, assuming in a specific embodiment a core 33, 1 inch in diameter and 27 inches long with a flame temperature 3,000" R, we have '1XPiX27 84.82

tions originating in the combustion chamber. In a specific embodiment in which the combustion chamber I2 has the dimensions described above, the exhaust tubes 13 would be 27 inches long and would have an internal diameter of 1% inches.

In order to extract additional thermal energy remaining in the exhaust gases afler passing along the lengths of the exhaust tubes 13, the temtinus of the exhaust system is designed to return these exhaust gases beneath the surface Bof the water A, catming said gases to bubble out from the surface into the atmosphere and to splash the water against heated metal parts of said terminus. For that purpose, there is connected to the upper end of each exhaust tube 13 the exhaust discharge devices 14, each comprising a 180 metal return bend 45,

adapted to extend above the water surface in operative in- I stalled position of the heater. The downwardly extending outlet end of this bend 45 is connected to a metal discharge pipe 46 depending downwardly therefrom into the water A, and extending at its lower open end below the surface B of said water. Around the bend 45 is a metal deflector hood 47 having a top wall 48 connected to the peak of the bend by means of a bolt 50, depending sidewalls 51 flanking the outer faces of said bend and extending at their lower sections below the surface of the water, and a depending rear end wall 52 also extending at its lower section below the surface of the water. The front end of the hood 47 is open.

The discharge pipe 46 has a discharge slot 53 below the surface of the water formed in its rear inner side nearest the rear wall 52 of the hood 47. This slot 53 is shown formed by a cut 5 54 in the discharge pipe 46 in a plane at right angles to the axis of said discharge pipe and cuts 55 converging downwardly discharge pipe. Part of the exhaust gas discharges from the slot 53, bubbles under water and is directed obliquely upwardly and rearwardly towards the bend 45 and towards the rear closed end of the hood 47, carrying with it water and splashing it with turbulence against the bend and the walls of the hood. This operation causes the water to absorb heat directly from the hot bend 45 and the hot hood 47. 4

During the periods of rarefactions in the pulse combustion cycles, the water tends to be syphoned through the lower ends of the discharge pipes 46 into the exhaust tubes 13. To prevent the water from being sucked back into the exhaust tubes 13, the bolt 50 securing the hood 4? 1' to the bend 45 has a passageway therethrough to serve also as a syphon breaker.

The extreme lower end of the discharge pipe 46 isfully open beneaththe surface of the water and tends to smooth out the surgesproduced by the pulsations in the combustion system.

FIG. 4 shows a modified form of fuel-air mixing device a in which the fuel gas is admitted into the throat region 25a of the venturi tube by means of a piezo ring 55 embedded in the ceramic lining 24a in said throat region and having an inlet 56. The fuel feed pipe 21 of FIG. 1 is eliminated, and in its stead, the solid electrode rod 27a extends along the full height of the fuel-air mixing device 100 through the insulating plug. 20a at the top closing the branch 17 of the tee 15.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

We claim:

l. A liquid heater of the liquid submersible type comprising a fuel-air mixing device with an outlet for discharge of combustible fuel-air mixtures, means forming a combustion chamber connected at one end to the outlet of said mixing device to receive said mixtures and comprising a heat-transmitting tubular wallenclosing said chamber, said chamber being submerged in the liquid to be heated in operative position of said heater, means for igniting the fuel-air mixtures in said chamber, an exhaust tube connected at one end to the other end of said chamber and extending along said chamber spaced-therefrom, said exhaust tube having a wall of heattransmitting material and being submerged in said liquid in operative position of said heater, and means at the other end of said exhaust tube for discharging the exhaust gases from said exhaust tube in said liquid below the surface of said liquid in operative position of said heater.

2. A liquid heater as described in claim 1, comprising a second exhaust tube connected at one end to said other end of said combustion chamber and extending along said chamber spaced therefrom, said second exhaust tube havinga wall of heat-transmitting material and being also submerged in said liquid in operative position of the heater, and a second means at the other end of said second exhaust tube for discharging the exhaust gases from said second exhaust tube in said liquid below the surface of said liquid in operative position of said heater.

3. A liquid heater as described in claim 1, said discharging means comprising conduit means having heat-transmitting walls located above the level of the liquid in operative position of the heater, and having a discharge pipe connected to the outlet of said conduit means and provided with a discharge opening in position to be submerged in the liquid in operative position of the heater, said discharge opening being located in position to cause liquid carried by the exhaust gases discharged from said discharge opening to be directed and splashed against the heat-transmitting walls of said conduit. means in operative position of the heater.

4. A liquid heater as described in claim 3, said conduit means comprising a metal return bend connected to said other end of said exhaust tube, said discharge pipe being connected to the outlet of said bend, and said discharge opening being located on the inner side of said discharge pipe nearest said exhaust tube.

5. A liquid heater as described in claim 4, comprising a hood of heat-transmitting material connected to said bend and enclosing said bend, said hood being open at one end, said discharge opening being positioned to direct the exhaust gases discharged from said discharge opening as well as the liquid carried by the latter gases against said bend and .said hood.

6. A liquid heater comprising a fuel-air mixing device including a venturi tube defining a throat region, means for admitting air into said tube on the upstream side of said throat region, and means for admitting fuel gas directly into the throat region of said tube, aided by the inspirating effect of the air passing through said throat to form fuel-air combustible mixtures, means forming a combustion chamber communicating with the discharge end of said tubetoreceiv said fuel-air mixtures and including a heat-transmitting tubular wall enclosing said chamber, and means for igniting'the mixtures in said chamber comprising an electrically. grounded screen across the downstream end of said tub'e ,,an insu lating spacer supported in said screen, and a spark-generating" electrode rod extending along said venturi tube and, passipgat its terminal end through said spacer and into said combustion chamber, said spacer separating the terminal spark-emitting end of said electrode rod and said screen by a spark gap,

7. A liquidheater as described in claim 6, said electrode rod having a tubular section provided with discharge port pieans at said throat region and constituting said means for admitting fuel gas into saidthroat region.

8. A liquid heater of the submersible type comprising means forming a combustion chamber and comprising a heat-transmitting tubular wall enclosing said chamber, said combustion chamber being submerged in the liquid to be heated in operative position of the heater, means for delivering combustible mixtures of fuel and air into said chamber and for igniting said mixtures, a body of material having high heat radiation potential, located in said chamber, said body being spaced from said tubular wall and having its surface exposed to combustion ac- 1 tion in said chamber to cause said body to be heated to a degree to emit radiant energy from said body to said wall to heat said wall, means for carrying on combustion in said chamber'as a succession of pulses, the ignition and initiation of combustion taking place in the headspace of said combustion chamber, said means-for carrying on pulse combustion comprising an annular screen in said headspace equally spaced peripherally from said tubular wall to prevent quenching of combustion in said headspace. I

9. A liquid heater of the submersible type comprising means forming acombustion chamber having a headspaceand comprising a heat-transmitting tubular wall enclosing said chamber, said combustion chamber being submerged-in the liquid to be heated in operative position of the heater, means for delivering combustible mixtures of fuel and air into said headspace, means for igniting and initially said mixtures in said headspace, means for carrying out combustion in and throughout said headspace as a regular sequence of pulses with alternate phases of lower and higher pressures, and comprising a rod of material having high heat radiation potential spaced from one end of said headspace to define the other end of said headspace and confining said headspace to a volume sufficiently restricted to permit generation of regular combustion pulses in said headspace, said rod extending from said headspace along said tubular wall and being peripherally spaced from said wall to form a space around a said rod .communicating with said headspace at said other end of said headspace, the end of said rod nearest said headspace being directly in the region of combustion in said headspace to cause some combustion in said headspace to take place directly on the surface of said rod end.

10. A liquid heater as described in claim 9, said means for igniting initially the combustible mixtures being adapted to be rendered inoperative after combustion in said headspace has been established, said rod being of a material to reignite the combustible mixture in the headspace in case combustion is interrupted in said headspace after said means for initially igniting the combustible mixture has been red rendered inoperative.

11. A liquid heater as described in claim 9, said means'for carrying out combustion in and throughout said headspace comprising means for continuing combustion in said space and along the surface of said rod along said space.

12. A liquid heater of the submersible type comprising means forming a combustion chamber having a headspace and comprising a heat-transmitting tubular wall enclosing said chamber, said combustion chamber being submerged in the liquid to be heated in operative position of the heater, means bular wall and being peripherally spaced from said wall to fonn a space around said rod communicating with said headspace at said end of said headspace, the end of said rod nearest said headspace being directly in the region of combustion in said headspace to cause some combustion in said headspace to take place directly on the surface of said rod end, the combustion in said headspace being continued in said space and over the peripheral surface of said rod. 

1. A liquid heater of the liquid submersible type comprising a fuel-air mixing device with an outlet for discharge of combustible fuel-air mixtures, means forming a combustion chamber connected at one end to the outlet of said mixing device to receive said mixtures and comprising a heat-transmitting tubular wall enclosing said chamber, said chamber being submerged in the liquid to be heated in operative position of said heater, means for igniting the fuel-air mixtures in said chamber, an exhaust tube connected at one end to the other end of said chamber and extending along said chamber spaced therefrom, said exhaust tube having a wall of heat-transmitting material and being submerged in said liquid in operative position of said heater, and means at the other end of said exhaust tube for discharging the exhaust gases from said exhaust tube in said liquid below the surface of said liquid in operative position of said heater.
 2. A liquid heater as described in claim 1, comprising a second exhaust tube connected at one end to said other end of said combustion chamber and extending along said chamber spaced therefrom, said second exhaust tube having a waLl of heat-transmitting material and being also submerged in said liquid in operative position of the heater, and a second means at the other end of said second exhaust tube for discharging the exhaust gases from said second exhaust tube in said liquid below the surface of said liquid in operative position of said heater.
 3. A liquid heater as described in claim 1, said discharging means comprising conduit means having heat-transmitting walls located above the level of the liquid in operative position of the heater, and having a discharge pipe connected to the outlet of said conduit means and provided with a discharge opening in position to be submerged in the liquid in operative position of the heater, said discharge opening being located in position to cause liquid carried by the exhaust gases discharged from said discharge opening to be directed and splashed against the heat-transmitting walls of said conduit means in operative position of the heater.
 4. A liquid heater as described in claim 3, said conduit means comprising a metal return bend connected to said other end of said exhaust tube, said discharge pipe being connected to the outlet of said bend, and said discharge opening being located on the inner side of said discharge pipe nearest said exhaust tube.
 5. A liquid heater as described in claim 4, comprising a hood of heat-transmitting material connected to said bend and enclosing said bend, said hood being open at one end, said discharge opening being positioned to direct the exhaust gases discharged from said discharge opening as well as the liquid carried by the latter gases against said bend and said hood.
 6. A liquid heater comprising a fuel-air mixing device including a venturi tube defining a throat region, means for admitting air into said tube on the upstream side of said throat region, and means for admitting fuel gas directly into the throat region of said tube, aided by the inspirating effect of the air passing through said throat to form fuel-air combustible mixtures, means forming a combustion chamber communicating with the discharge end of said tube to receive said fuel-air mixtures and including a heat-transmitting tubular wall enclosing said chamber, and means for igniting the mixtures in said chamber comprising an electrically grounded screen across the downstream end of said tube, an insulating spacer supported in said screen, and a spark-generating electrode rod extending along said venturi tube and passing at its terminal end through said spacer and into said combustion chamber, said spacer separating the terminal spark-emitting end of said electrode rod and said screen by a spark gap.
 7. A liquid heater as described in claim 6, said electrode rod having a tubular section provided with discharge port means at said throat region and constituting said means for admitting fuel gas into said throat region.
 8. A liquid heater of the submersible type comprising means forming a combustion chamber and comprising a heat-transmitting tubular wall enclosing said chamber, said combustion chamber being submerged in the liquid to be heated in operative position of the heater, means for delivering combustible mixtures of fuel and air into said chamber and for igniting said mixtures, a body of material having high heat radiation potential, located in said chamber, said body being spaced from said tubular wall and having its surface exposed to combustion action in said chamber to cause said body to be heated to a degree to emit radiant energy from said body to said wall to heat said wall, means for carrying on combustion in said chamber as a succession of pulses, the ignition and initiation of combustion taking place in the headspace of said combustion chamber, said means for carrying on pulse combustion comprising an annular screen in said headspace equally spaced peripherally from said tubular wall to prevent quenching of combustion in said headspace.
 9. A liquid heater of the submersible type comprising means forming a combustion chambEr having a headspace and comprising a heat-transmitting tubular wall enclosing said chamber, said combustion chamber being submerged in the liquid to be heated in operative position of the heater, means for delivering combustible mixtures of fuel and air into said headspace, means for igniting and initially said mixtures in said headspace, means for carrying out combustion in and throughout said headspace as a regular sequence of pulses with alternate phases of lower and higher pressures, and comprising a rod of material having high heat radiation potential spaced from one end of said headspace to define the other end of said headspace and confining said headspace to a volume sufficiently restricted to permit generation of regular combustion pulses in said headspace, said rod extending from said headspace along said tubular wall and being peripherally spaced from said wall to form a space around a said rod communicating with said headspace at said other end of said headspace, the end of said rod nearest said headspace being directly in the region of combustion in said headspace to cause some combustion in said headspace to take place directly on the surface of said rod end.
 10. A liquid heater as described in claim 9, said means for igniting initially the combustible mixtures being adapted to be rendered inoperative after combustion in said headspace has been established, said rod being of a material to reignite the combustible mixture in the headspace in case combustion is interrupted in said headspace after said means for initially igniting the combustible mixture has been red rendered inoperative.
 11. A liquid heater as described in claim 9, said means for carrying out combustion in and throughout said headspace comprising means for continuing combustion in said space and along the surface of said rod along said space.
 12. A liquid heater of the submersible type comprising means forming a combustion chamber having a headspace and comprising a heat-transmitting tubular wall enclosing said chamber, said combustion chamber being submerged in the liquid to be heated in operative position of the heater, means for delivering combustible mixtures of fuel and air into said headspace, means for igniting said mixtures in said headspace, a rod of material having high radiation potential spaced from one end of said headspace to define the other end of said headspace and confining said headspace to a volume sufficiently restricted to cause combustion in said headspace to take place throughout said headspace and to an incomplete extent, said rod extending from said headspace along said tubular wall and being peripherally spaced from said wall to form a space around said rod communicating with said headspace at said end of said headspace, the end of said rod nearest said headspace being directly in the region of combustion in said headspace to cause some combustion in said headspace to take place directly on the surface of said rod end, the combustion in said headspace being continued in said space and over the peripheral surface of said rod. 